This project is directed toward using nucleic acid hybridization techniques to examine human neoplastic cells for the presence of nucleic acids related to RNA genomes of mammalian RNA tumor viruses. Both the DNA-excess method for hybridization and the competition hybridization assay in DNA-excess have been employed to analyze four human high-molecular-weight (HMW) RNAs released by fibrosarcoma, osteosarcoma, human fetal prostate, and benign prostate hyperplasia cell cultures. A comparison of the ability of these HMW RNAs to cross-hybridize with viral complementary DNA transcripts revealed that the HMW RNA released by the human fetal prostate culture exhibited less sequence homology with the viral probes than did the other three HMW RNAs. The results also indicated that the latter three HMW RNAs cross-hybridized to higher levels with DNA transcripts of Moloney murine sarcoma virus, Kirsten murine sarcoma virus, and Kirsten murine leukemia virus than with those of Rauscher murine leukemia virus, Gross murine leukemia virus, and simian sarcoma virus. In another investigation, the competition hybridization assay was used to measure the intracellular distribution of RNA genomes of Rauscher leukemia virus. Also, intracellular RNA which is complementary in base sequence to the RNA genome of Moloney sarcoma virus was detected in virus-producing rat cells.